This invention relates to an optimal acoustic wave propagation orientation on a Lithium Tantalate crystal for surface acoustic wave (SAW) applications including natural single-phase unidirectional transducers. SAW devices are used for a large number of applications including impedance elements, resonators, coupled resonator filters, transversal filters, and the like.
Surface acoustic wave technology for signal processing has produced a broad range of devices which are typically used in the VHF and UHF range and include filters, resonators, delay lines, convolvers, impedance elements and various other types of devices. Processes for preparing these devices are well known in the art and the devices typically involve interdigitated electrodes.
The orientations for surface acoustic wave propagation for any given crystal are completely defined by the Euler angles, lambda, mu and theta. The first two angles lambda and mu, define the crystal cut and the third angle, theta defines the direction of acoustic wave propagation in that cut. Thus, acoustic wave propagation orientation in the crystal is defined by a unique set of all three Euler angles. See Holstein, H., Classical Mechanics, N.Y., (1950) (Addison-Wesley).
In locating desired crystal cuts for surface acoustic wave device applications, the concern in the prior art is to find a crystal orientation which has good coupling between the voltage on the electrodes and acoustic wave in the crystal itself, low beam steering, and good temperature stability. This criterion was used to generate a theoretical tabulation of surface acoustic wave properties at various orientations for a variety of surface wave materials and particular crystal cuts of interest. This tremendous work was undertaken by Slobodnik et al and can be found in Microwave Acoustic Handbook 1A and 2, Air Force Cambridge Research Labs, October 1973 and October 1974.
In 1985 Peter V. Wright disclosed his discovery of the natural single phase unidirectional transducer, "The Natural Single Phase Unidirectional Transducer, A New Low Loss SAW Structure", 1985 Ultrasonics Symposium Proceedings, pp. 58-63. Wright found that by selecting crystal cuts using a criterion in addition to and not considered by Slobodnik et al, he was able to introduce a unidirectionality into SAW transducers employing 2 electrodes per wavelength. The unidirectionality is the direct result of introducing a separation between the centers of transduction and reflection. The desired separation, as set forth in U.S. Pat. No. 4,599,587, is 45.degree.. Wright's discovery subsequently led to U.S. Pat. No. 4,670,681, issued to Wright on June 2, 1987 and U.S. Pat. No. 4,670,680, issued to Andle on June 2, 1987. In U.S. Pat. No. 4,670,681, Wright disclosed a crystal cut of Quartz suitable for natural single phase unidirectional transducer applications. In U.S. Pat. No. 4,670,680, Andle disclosed a second crystal cut of Quartz suitable for natural single phase unidirectional transducer applications.
The inventors have discovered that a Lithium Tantalate crystal having a cut defined by the Euler angles of lambda equal about 0.degree., mu equal about 90.degree. and theta equals about plus 141.25.degree., an orientation which has not been heretofore disclosed, provides the needed advantages as set forth above when electrodes are placed on a Lithium Tantalate crystal. Additionally, the discovered crystal cut combines the criterion for implementation of natural single phase unidirectional transducers with an increased coupling between the potential on the electrodes and the surface waves in the crystal. This advantage of increased coupling allows for the implementation of SAW devices with fractional bandwidths larger than those which can be implemented using the crystal cuts disclosed in U.S. Pat. Nos. 4,670,680 and 4,670,681. It also allows the construction of a two electrode per wavelength transducer with a desired symmetrical input conductance function and a flat susceptance region. See U.S. Pat. No. 4,599,587, issued July 8, 1986 and entitle Impedance Element for a detailed description of the desired conductance and susceptance.